The concept of triploid (seedless) watermelons was first described in the U.S. literature by Kihara in 1951 based on experimentation that began in 1939 in Japan. Seed for planting seedless watermelons results from a cross between a tetraploid female parent, developed by treating diploid lines with colchicine or by other means, and a diploid (normal) male parent. The resulting triploid plants are sterile and do not produce viable seed. However, small, rudimentary seeds develop which are eaten along with the flesh just as immature seeds are eaten in cucumber.

Fruit enlargement in seeded fruit, including watermelon, is enhanced by growth-promoting hormones produced by the developing seed. Growth hormones are lacking in seedless watermelons so those agents must be provided by pollen. Since flowers of triploid plants lack sufficient viable pollen to induce normal fruit set, diploid seeded watermelons are interplanted with triploids to serve as pollenizers. An adequate bee population is necessary to insure that sufficient transfer of pollen occurs. Seedless fruit (from triploid plants) tend to be triangular shaped without sufficient pollination.

Although the procedure for production of seedless watermelons has been known for about 50 years and commercial varieties have been available for many years, the interest in and acreage of seedless watermelons has remained small in Florida until recently. Erratic performance, poor seed germination, high seed costs, and inadequate varieties resulted in lack of interest in seedless watermelon production in the past, but most of the deterents have now been overcome. It is estimated that seedless watermelons now represent about 30% of the total production in Florida.

The objective of this trial was to evaluate the performance of triploid watermelon cultigens under west-central Florida conditions.

Seeds of 37 triploid watermelon varieties or experimental hybrids (Table 1) were planted in a peat-lite growing mix in planter flats (1 ¼ x 1 ¼ x 2 ¼ in. cells) on 23 January. The watermelon transplants were grown by a commercial plant grower.

The EauGallie fine sand was prepared in late January by incorporation of 0-0.8-0 lb N-P₂O₅-K₂O per 100 linear bed feet (lbf). Beds were formed and fumigated with methylbromide:chloropicrin, 67:33 at 2.3 lb/100 lbf. Banded fertilizer was applied in shallow grooves on the bed shoulders at 3.1-0-4.3 lb N-P₂O₅-K₂O/100 lbf after the beds were pressed and before the black polyethylene mulch was applied. The total fertilizer applied was equivalent to 150-40-208 lb N-P₂O₅-K₂O/A. The final beds were 32-in. wide and 8-in. high, and were spaced on 9-ft centers with four beds between seepage irrigation/drainage ditches, which were on 41-ft centers.

The transplants were set in holes punched in the polyethylene mulch at 3-ft in-row spacing on 26 February. The replicated plots were 27 ft long and had nine plants each and were repeated three times in a randomized, complete block design. Diploid watermelons that were being evaluated were direct seeded in beds on each side of two triploid watermelon beds on 19 February to serve as diploid pollenizers. Plant stands recorded just before vines grew together showed no significant differences among plots. Weed control in row middles was by cultivation and applications of paraquat. Pesticides were applied as needed for control of silverleaf whitefly (endosulfan), gummy stem blight (chlorothalonil, azoxystrobin, mefenoxam, maneb, and thiophanate-methyl), and lepidopterous larvae (Bacillus thuringiensis, spinosad, and esfenvalerate).

Watermelons were harvested during the 24-30 May and 6-12 June periods. Marketable (U.S. No.1 or better) fruit according to U.S. Standards for Grades of Watermelons were separated from culls and counted and weighed individually. Fruit 10 lbs and larger were assumed to be marketable. Tetraploid fruit, where they occurred, were not included in the marketable category because they are not seedless. At least six fruit from each entry at each harvest were used to determine soluble solids (a measure of sweetness) with a digital, hand-held refractometer, polar and equatorial dimensions, rind thickness, flesh color, and the incidence and severity of hollowheart were noted.

Early yield, as represented by the first of two harvests, varied from 95 cwt/acre for Hazera 103 to 550 cwt/acre for ‘Millionaire’ (Table 1). Nineteen other entries had yields similar to those of ‘Millionaire’. Average fruit weights at the first harvest ranged from 13.9 lbs for ‘Imagination’ to 21.7 lbs for Seedway 4502.

Total yields (Table 1) ranged from 567 cwt/acre for ‘Revolution’ to 1149 cwt/acre for ‘Millionaire’. Twenty-three other entries produced yields significantly similar to those of ‘Millionaire’. Average fruit weight for the entire season varied from 14.5 lbs for ‘Imagination’ to 21.7 lbs ‘Revolution’. The number of fruit per plant ranged from 1.7 for ‘Revolution’ to 4.3 for ‘Millionaire’. Soluble solids concentrations varied from 12.0% for Seedway 4502 to 14.1% for ZG-8820. Accordingly, soluble solids in all entries far exceeded the 10% specified for optional use in the U.S. Standards for Grades of Watermelons to describe very good internal quality. The incidence of hollowheart in the fruit sampled ranged from 0% in ‘Super Seedless 7177’, ‘Tri-X Palomar’, ‘Trillion’ and ‘Super Seedless 7187’ to 58% in ‘Triple Crown’. Tetraploid plants were noted in several entries. The total number of tetraploid fruit produced follow: ‘Cooperstown’ (6), ‘Fandango’ (3), ‘Genesis’ (2), HMX 8914 (5), Seedway 4502 (5), ‘Sugar Shack’ (9), ‘Sugar Time’ (1), SWT 9708 (2). These fruit could not be sold as seedless watermelons so they were not included in the marketable yields.

Seedless watermelon variety trials have been conducted at this location each spring season since 1988. The highest yields ranged from 507 cwt/acre in 1996 to 1186 cwt/A in 1999. In spring 2001, 1149 cwt/acre was the highest yield which greatly exceeded the 844 cwt/acre average high yield during the entire period.

Based on results of this and previous trials, triploid hybrids, in alphabetical order, that should perform well in Florida include ‘Freedom’, ‘Genesis’, ‘Millionaire’, ‘Revere’, ‘Sugar Shack’ (for trial), ‘Sugar Time’ (for trial), ‘Super Seedless 7177’ (for trial), ‘Summersweet 5244’, ‘Summersweet 5544’, ‘Tri-X 313’, ‘Tri-X Carousel’, ‘Tri-X Palomar’, and ‘Tri-X Shadow’. ‘Triton’, a yellow-flesh variety should be evaluated for that niche market. Other varieties may perform well on individual farms.

A complete report of this trial can be obtained from the author at dnma@mail.ifas.ufl.edu

 (Maynard - Vegetarian 01-11)